A known related-art normally closed three-port valve equipped with the above-described balancing mechanism includes a valve body having a basic structure similar to the three-port valve according to the present invention. In this related-art normally closed three-port valve, a pressure of a fluid having flowed to an output port side when a valve was opened by pressing a rod for operating a valve member is caused to operate as a back pressure for closing the valve applied to the valve member.
In the case of a normally closed three-port valve without the balancing mechanism, the pressure of the fluid having flowed to the output port side when the valve was opened by pressing the valve member with the rod operates as a force in a direction in which the valve member is held at the valve open position. Thus, even when a pressing force for opening the valve applied to the valve opening rod is lost, the valve is not closed until a valve closing force against the pressure of the fluid having flowed to the output port side acts on the valve member. For example, it is required that the valve closing force be applied by a return spring for the valve member or a mechanism that forcibly closes the valve be provided. When the valve closing force is caused to act on the valve member by the return spring, in order to open the valve, it is required that the valve be opened against an urging force of the return spring and the pressure of a main fluid. Accordingly, in order to open the valve, a large operating force is required to be applied to the rod.
In order to reduce the operating force for open the valve, the balancing mechanism balances the fluid pressures acting on the front and back surfaces of the valve member by causing the pressure of the main fluid on the output port side to flow into a back pressure chamber formed at the back of the valve member through a vent hole provided in the valve member when the valve member is at the valve open position. In order to open the valve, it is required that a discharge channel open to the outside through the rod be closed, and at the same time, the main valve seat be opened by the valve member, and the vent hole of the valve member for causing the pressure of the main fluid on the output port side to flow into the back pressure chamber be held in a state in which the vent hole is open to the output port side. This three-port valve, which is typically closed by a return spring applying a small urging force acting on the valve member, is closed by releasing the pressure applied to the valve member by the rod, and at the same time, opening the discharge channel open to the outside through the rod.
Thus, a known normally closed three-port valve typically uses a valve member 40 having a structure as illustrated in FIG. 8. In use, this valve member 40 is attached to a valve body 1 having a structure similar to a normally closed three-port valve illustrated in FIG. 1. A valve member main body 41 faces a main valve seat 2 of the valve body 1 and is airtightly held such that the valve member main body 41 is slidable in a contacting and separating direction. The valve member main body 41 has a main seat surface 44 and a discharge seat surface 45. The main seat surface 44 is brought into contact with and separated from the main valve seat 2 of the valve body 1. The discharge seat surface 45 is brought into pressure contact with an inner end of a discharge channel 6 of the valve opening rod 5 so as to close the discharge channel 6. It is originally preferable that the discharge seat surface 45 be disposed at a position immediately adjacent to the main seat surface 44 around the main seat surface 44. However, since the vent hole that communicates with the back pressure chamber 8 is required to open at a region between both the seat surfaces, both the main seat surface 44 and the discharge seat surface 45 cannot be provided on the same plane. Thus, a level difference 42 is provided on an upper portion of the valve member main body 41, and the main seat surface 44 is provided on a lower level and the discharge seat surface 45 is provided on an upper level. This allows the vent hole to be open in the level difference 42, and accordingly, the fluid can stably flow and be blocked by both the seat surface portions 44 and 45.
However, when the above-described structure is adopted, it is difficult to independently stably secure the main seat surface 44 and the discharge seat surface 45 to the valve member main body 41 after the main seat surface 44 and the discharge seat surface 45 have been formed as independent seat members in the manufacture of the valve member 40. Accordingly, it is required that, as a single rubber lining formed of a seat member having rubber elasticity, a seat member 43 that extends through the upper and lower levels of the level difference 42 be provided in the valve member main body 41. It is also required that, in order to open the vent hole at a side surface portion of the level difference 42, a vent hole 46, which does not reach from a lower surface to an upper surface of the valve member main body 41, and a branched vent hole 47, through which an upper portion of the vent hole 46 is open to the side surface, be formed. Thus, it is difficult to make the valve member main body 41 by molding, and accordingly, it is required that a structure for lining the seat member 43, the vent hole 46, and the branched vent hole 47 be formed by cutting a metal material. Furthermore, the valve member main body 41 is required to be separately lined with rubber. As a result, the manufacturing cost of the valve member becomes comparatively high.